A dye-sensitized solar cell of a new type reported in a journal “Nature” in 1991 by Graetzel et al. received attention as exhibiting a remarkably high conversion efficiency (on the order of 7%) compared with conventional dye-sensitized solar cells. A dye-sensitized solar cell realizes a photoelectric conversion by injecting excited electrons generated by a dye that has collected light to a semiconductor. Thus, it is important that a large amount of sensitizing dye is carried on a semiconductor so as to enhance a light collecting ability, and electrons are injected to a semiconductor as fast as possible from the sensitizing dye. The new dye-sensitized solar cell that also is called a Graetzel cell solves this problem by allowing a porous film made of titanium oxide of ultrafine particles to carry a ruthenium complex that is a sensitizing dye.
The Graetzel cell can be assembled merely by coating a transparent electrode with a paste in which ultrafine particles of titanium oxide are dispersed, allowing the transparent electrode to carry a sensitizing dye, and fling an electrolyte between the transparent electrode and a counter electrode. Compared with conventional solar cells, the Graetzel cell can be produced with a simple apparatus, so that it receives attention as one of the next generation solar cells.
A major feature of the Graetzel cell is to use a porous semiconductor film obtained by sintering titanium oxide of ultrafine particles. The purpose of sintering titanium oxide is to allow ultrafine particles of a semiconductor to bind each other, and to ensure a conducting path for optically excited electrons injected from a sensitizing dye. Usually, the sintering temperature of titanium oxide for ensuring a conducting path for optically excited electrons is in a range of 450° C. to 550° C. When the sintering temperature is less than this range, the binding between the ultrafine particles of a semiconductor becomes insufficient. Because of this, unless a material having a softening temperature higher than the sintering temperature is selected as a substrate of a transparent electrode for forming a porous titanium oxide film, the transparent electrode actually cannot be used. However, most of the materials having light transparency have a softening temperature lower than the sintering temperature of titanium oxide. Therefore, it is difficult to use such a material as an electrode substrate of the Graetzel cell.
Furthermore, when a film is used as a substrate of the Graetzel cell, for example, a roll-to-roll continuous manufacturing process described in WO 97/15959 and a production method suitable for mass-production described in WO 99/66519 can be adopted, and the Graetzel cell can be produced at a lower cost than the existing solar cells. Therefore, the film type Graetzel cell can be deployed for a very wide use. However, when a film is used as a substrate, a porous titanium oxide film made of ultrafine particles cannot handle the flexibility of the film, and becomes likely to crack or peel off. Furthermore, in WO 93/20569, a method for adding a surfactant “TRITON X-100” of a nonionic type to a titanium oxide paste for the purpose of reducing cracking of a coating film during coating of the titanium oxide paste is described. In this method, as much as 40% by mass of “TRITON X-100” is added to titanium oxide, which may inhibit the transfer of electrons in a titanium oxide film.
Furthermore, in WO 00/72373, by applying a pressure of 100 to 1000 kg/cm2 to a titanium oxide film, the mechanical strength and electron conducting path of the titanium oxide film are ensured without sintering titanium oxide. This technique is characterized in that a binder is not contained in a titanium oxide film so as to avoid the inhibition of electron transfer in a titanium oxide film by a binder.
On the other hand, the inventors of the present invention confirmed that a titanium oxide film with mechanical strength to some degree can be obtained by applying a large pressure to the titanium oxide film. However, the film thus obtained has weak adhesion with respect to a substrate, and the titanium oxide film is likely to peel off.